Tropical Cloud Systems Research Articles

Convective organization and 3D structure of tropical cloud systems deduced from synergistic A-Train observations and machine learning

Abstract. We are building a 3D description of upper tropospheric (UT) cloud systems in order to study the relation between convection and cirrus anvils. For this purpose we used cloud data from the Atmospheric InfraRed Sounder and the Infrared Atmospheric Sounding Interferometer and atmospheric and surface properties from the meteorological reanalyses ERA-Interim and machine learning techniques. The different artificial neural network models were trained on collocated radar–lidar data from the A-Train in order to add cloud top height, cloud vertical extent and cloud layering, as well as a rain intensity classification to describe the UT cloud systems. The latter has an accuracy of about 65 % to 70 % and allows us to build objects of strong precipitation, used to identify convective organization. This rain intensity classification is more efficient to detect large latent heating than cold cloud temperature. In combination with a cloud system analysis, we found that deeper convection leads to larger heavy rain areas and a larger detrainment, with a slightly smaller thick anvil emissivity. This kind of analysis can be used for a process-oriented evaluation of convective precipitation parameterizations in climate models. Furthermore, we have shown the usefulness of our data to investigate tropical convective organization metrics. A comparison of different tropical convective organization indices and proxies to define convective areas has revealed that all indices show a similar annual cycle in convective organization, in phase with convective core height and anvil detrainment. The geographical patterns and magnitudes in radiative heating rate interannual changes with respect to one specific convective organization index (Iorg) for the period 2008 to 2018 are similar to the ones related to the El Niño–Southern Oscillation. However, since the interannual anomalies of the convective organization indices are very small and noisy, it was impossible to find a coherent relationship with those of other tropical mean variables such as surface temperature, thin cirrus area or subsidence area.

The temperature dependence of ice-nucleating particle concentrations affects the radiative properties of tropical convective cloud systems

Abstract. Convective cloud systems in the maritime tropics play a critical role in global climate, but accurately representing aerosol interactions within these clouds persists as a major challenge for weather and climate modelling. We quantify the effect of ice-nucleating particles (INPs) on the radiative properties of a complex tropical Atlantic deep convective cloud field using a regional model with an advanced double-moment microphysics scheme. Our results show that the domain-mean daylight outgoing radiation varies by up to 18 W m−2 depending on the chosen INP parameterisation. The key distinction between different INP parameterisations is the temperature dependence of ice formation, which alters the vertical distribution of cloud microphysical processes. The controlling effect of the INP temperature dependence is substantial even in the presence of Hallett–Mossop secondary ice production, and the effects of secondary ice formation depend strongly on the chosen INP parameterisation. Our results have implications for climate model simulations of tropical clouds and radiation, which currently do not consider a link between INP particle type and ice water content. The results also provide a challenge to the INP measurement community, as we demonstrate that INP concentration measurements are required over the full mixed-phase temperature regime, which covers around 10 orders of magnitude.

Atmospheric energy budget response to idealized aerosol perturbation in tropical cloud systems

Abstract. The atmospheric energy budget is analysed in numerical simulations of tropical cloud systems to better understand the physical processes behind aerosol effects on the atmospheric energy budget. The simulations include both shallow convective clouds and deep convective tropical clouds over the Atlantic Ocean. Two different sets of simulations, at different dates (10–12 and 16–18 August 2016), are simulated with different dominant cloud modes (shallow or deep). For each case, the cloud droplet number concentration (CDNC) is varied as a proxy for changes in aerosol concentrations without considering the temporal evolution of the aerosol concentration (for example due to wet scavenging, which may be more important under deep convective conditions). It is shown that the total column atmospheric radiative cooling is substantially reduced with CDNC in the deep-cloud-dominated case (by ∼10.0 W m−2), while a much smaller reduction (∼1.6 W m−2) is shown in the shallow-cloud-dominated case. This trend is caused by an increase in the ice and water vapour content at the upper troposphere that leads to a reduced outgoing longwave radiation, an effect which is stronger under deep-cloud-dominated conditions. A decrease in sensible heat flux (driven by an increase in the near-surface air temperature) reduces the warming by ∼1.4 W m−2 in both cases. It is also shown that the cloud fraction response behaves in opposite ways to an increase in CDNC, showing an increase in the deep-cloud-dominated case and a decrease in the shallow-cloud-dominated case. This demonstrates that under different environmental conditions the response to aerosol perturbation could be different.

A representation of the collisional ice break-up process in the two-moment microphysics LIMA v1.0 scheme of Meso-NH

Abstract. The paper describes a switchable parameterization of collisional ice break-up (CIBU), an ice multiplication process that fits in with the two-moment microphysical Liquid Ice Multiple Aerosols (LIMA) scheme. The LIMA scheme with three ice types (pristine cloud ice crystals, snow aggregates, and graupel hail) was developed in the cloud-resolving mesoscale model (Meso-NH). Here, the CIBU parameterization assumes that collisional break-up is mostly efficient for the small and fragile snow aggregate class of particles when they are hit by large, dense graupel particles. The increase of cloud ice number concentration depends on a prescribed number (or a random number) of fragments being produced per collision. This point is discussed and analytical expressions of the newly contributing CIBU terms in LIMA are given. The scheme is run in the cloud-resolving mesoscale model (Meso-NH) to simulate a first case of a three-dimensional deep convective event with heavy production of graupel. The consequence of dramatically changing the number of fragments produced per collision is investigated by examining the rainfall rates and the changes in small ice concentrations and mass mixing ratios. Many budgets of the ice phase are shown and the sensitivity of CIBU to the initial concentration of freezing nuclei is explored. The scheme is then tested for another deep convective case where, additionally, the convective available potential energy (CAPE) is varied. The results confirm the strong impact of CIBU with up to a 1000-fold increase in small ice concentrations, a reduction of the rainfall or precipitating area, and an invigoration of the convection with higher cloud tops. Finally, it is concluded that the efficiency of the ice crystal fragmentation needs to be tuned carefully. The proposed parameterization of CIBU is easy to implement in any two-moment microphysics scheme. It could be used in this form to simulate deep tropical cloud systems where anomalously high concentrations of small ice crystals are suspected.

Shallow Circulations: Relevance and Strategies for Satellite Observation

Shallow circulations are central to many tropical cloud systems. We investigate the potential of existing and upcoming data to document these circulations. Different methods to observe or constrain atmospheric circulations rely on satellite-borne instruments. Direct observations of the wind are currently possible at the ocean surface or using tracer patterns. Satellite-borne wind lidar will soon be available, with a much better coverage and accuracy. Meanwhile, circulations can be constrained using satellite observations of atmospheric diabatic heating. We evaluate the commonalities and discrepancies of these estimates together with reanalysis in systems that include shallow circulations. It appears that existing datasets are in qualitative agreement, but that they still differ too much to provide robust evaluation criteria for general circulation models. This state of affairs highlights the potential of satellite-borne wind lidar and of further work on current satellite retrievals.

A global view of atmospheric ice particle complexity

AbstractAtmospheric ice particles exist in a variety of shapes and sizes. Single hexagonal crystals like common hexagonal plates and columns are possible, but more frequently, atmospheric ice particles are much more complex. Ice particle shapes have a substantial impact on many atmospheric processes through fall speed, affecting cloud lifetime, to radiative properties, affecting energy balance to name a few. This publication builds on earlier work where a technique was demonstrated to separate single crystals and aggregates of crystals using particle imagery data from aircraft field campaigns. Here data from 10 field programs have been analyzed and ice particle complexity parameterized by cloud temperature for arctic, midlatitude (summer and frontal), and tropical cloud systems. Results show that the transition from simple to complex particles can be as small as 80 µm or as large as 400 µm depending on conditions. All regimes show trends of decreasing transition size with decreasing temperature.

Retrieval of radiative and microphysical properties of clouds from multispectral infrared measurements

Satellite remote sensing of the macroscopic, microphysical, and optical properties of clouds are useful for studying spatial and temporal variations of clouds at various scales and constraining cloud physical processes in climate and weather prediction models. Instead of using separate independent algorithms for different cloud properties, a unified, optimal estimation-based cloud retrieval algorithm is developed and applied to moderate resolution imaging spectroradiometer (MODIS) observations using ten thermal infrared bands. The model considers sensor configurations, background surface and atmospheric profile, and microphysical and optical models of ice and liquid cloud particles and radiative transfer in a plane-parallel, multilayered atmosphere. Measurement and model errors are thoroughly quantified from direct comparisons of clear-sky observations over the ocean with model calculations. Performance tests by retrieval simulations show that ice cloud properties are retrieved with high accuracy when cloud optical thickness (COT) is between 0.1 and 10. Cloud-top pressure is inferred with uncertainty lower than 10 % when COT is larger than 0.3. Applying the method to a tropical cloud system and comparing the results with the MODIS Collection 6 cloud product shows good agreement for ice cloud optical thickness when COT is less than about 5. Cloud-top height agrees well with estimates obtained by the CO2 slicing method used in the MODIS product. The present algorithm can detect optically thin parts at the edges of high clouds well in comparison with the MODIS product, in which these parts are recognized as low clouds by the infrared window method. The cloud thermodynamic phase in the present algorithm is constrained by cloud-top temperature, which tends not to produce results with an ice cloud that is too warm and liquid cloud that is too cold.

Comparisons of GCM cloud cover parameterizations with cloud-resolving model explicit simulations

Three kinds of the widely-used cloudiness parameterizations are compared with data produced from the cloud-resolving model (CRM) simulations of the tropical cloud system. The investigated schemes include those based on relative humidity (RH), the semi-empirical scheme using cloud condensate as a predictor, and the statistical scheme based on probability distribution functions (PDFs). Results show that all three schemes are successful in reproducing the timing of cloud generation, except for the RH-based scheme, in which low-level clouds are artificially simulated during cloudless days. In contrast, the low-level clouds are well simulated in the semi-empirical and PDF-based statistical schemes, both of which are close to the CRM explicit simulations. In addition to the Gaussian PDF, two alternative PDFs are also explored to investigate the impact of different PDFs on cloud parameterizations. All the PDF-based parameterizations are found to be inaccurate for high cloud simulations, in either the magnitude or the structure. The primary reason is that the investigated PDFs are symmetrically assumed, yet the skewness factors in deep convective cloud regimes are highly significant, indicating the symmetrical assumption is not well satisfied in those regimes. Results imply the need to seek a skewed PDF in statistical schemes so that it can yield better performance in high cloud simulations.

A study of macrophysical and microphysical properties of warm clouds over the Northern Hemisphere using CloudSat/CALIPSO data

This study investigates the general macrophysical and microphysical properties of single-layer warm clouds over the Northern Hemisphere using standard CloudSat products during 2008. The yearly averaged occurrence frequency of single-layer warm clouds is 20.9% over oceans and 9.5% over land, respectively. The cloud top heights over land in different latitude zones exhibit a broader spectral width and more frequent occurrence of strong cloud systems in tropics than that over higher latitudes. The maximum values of liquid water content over land are approximately 10–30% smaller but the occurring altitudes are about 0.5 km higher than those over oceans with the same liquid water path (LWP). The reflectivity profiles clearly demonstrate the growth processes of cloud particles and how these processes vary with increasing LWP. The evolutions of reflectivity over land show a frequency shift toward weaker dBZ values relative to that over oceans with the same LWP, suggesting that high aerosol concentrations may induce suppressed drizzle and delayed precipitation in warm clouds. Additionally, there exists a significant difference between warm clouds formation at the initial stage over ocean and land areas. When LWP is 0.1–0.2 kg m−2 and optical depth exceeds 25, the dBZ values over oceans increase rapidly downward with increasing optical depth (decreasing height). The faster growth may be caused by the evaporation-condensation mechanism. Over land, however, large droplets are unlikely to appear in the early stage possibly because continental aerosols prevent the evaporation-condensation mechanism to occur, resulting in partially delayed drizzle formation.

Aerosol effects on the cloud-field properties of tropical convective clouds

Abstract. Aerosol effects on condensed water and precipitation in a tropical cloud system driven by deep convective clouds are investigated for two-dimensional simulations of 2-day duration. Although an assumed 10-fold increase in aerosol concentration results in a similar temporal evolution of mean precipitation and a small (9%) difference in cumulative precipitation between the high- and low-aerosol cases, the characteristics of the convection are much more sensitive to aerosol. The convective mass flux, and temporal evolution and frequency distribution of the condensed water path WP (sum of liquid- and ice-water paths) differ significantly between unperturbed and aerosol-perturbed simulations. There are concomitant differences in the relative importance of individual microphysical processes and the frequency distribution of the precipitation rate (P). With increasing aerosol (i) the convective mass flux increases, leading to increases in condensation, cloud liquid, and accretion of cloud liquid by precipitation; (ii) autoconversion of cloud water to rain water decreases; (iii) the WP spatial distribution becomes more homogeneous; and (iv) there is an increase in the frequencies of high and low WP and P, and a decrease in these frequencies at the mid-range of WP and P. Thus, while aerosol perturbations have a small influence on total precipitation amount, for the case considered, they do have substantial influence on the spatiotemporal distribution of convection and precipitation.

ARM Research In The Equatorial Western Pacific: A Decade And Counting

The tropical western Pacific (TWP) is an important climatic region. Strong solar heating, warm sea surface temperatures, and the annual progression of the intertropical convergence zone (ITCZ) across this region generate abundant convective systems, which through their effects on the heat and water budgets have a profound impact on global climate and precipitation. In order to accurately evaluate tropical cloud systems in models, measurements of tropical clouds, the environment in which they reside, and their impact on the radiation and water budgets are needed. Because of the remote location, ground-based datasets of cloud, atmosphere, and radiation properties from the TWP region have come primarily from shortterm field experiments. While providing extremely useful information on physical processes, these short-term datasets are limited in statistical and climatological information. To provide longterm measurements of the surface radiation budget in the tropics and the atmospheric properties that affect it, the Atmospheric Radiation Measurement program established a measurement site on Manus Island, Papua New Guinea, in 1996 and on the island republic of Nauru in late 1998. These sites provide unique datasets now available for more than 10 years on Manus and Nauru. This article presents examples of the scientific use of these datasets including characterization of cloud properties, analysis of cloud radiative forcing, model studies of tropical clouds and processes, and validation of satellite algorithms. New instrumentation recently installed at the Manus site will provide expanded opportunities for tropical atmospheric science.

Segmentation of Tropical Cyclone Cloud Systems with Compositive Algorithms

The North-West Pacific Ocean is one of the places that have the maximal frequency of the Tropical Cyclones(TC) and the highest intensity of TCs in the world,especially to which intensity level up to typhoon.Typhoon is one of the disaster weather systems of serious influence the mankind produce living,while our country is one of the stricken serious country that is subjected to it.Typhoon brings the very great threaten and disasters to industry and agriculture production,traffic and transportation,the safe of life and property of people in cities of near southeast of sea in our country.It is significant to investigate the characteristics of TCs in the North-West Pacific Ocean.With the development of detecting technique,nephogram became an important tool for monitoring tropical cyclone and typhoon in the tropical ocean,effective classification of image data and the segmentation of TC cloud are key steps for subsequent image feature extraction.Numerous scientific research had been launching into the area,compositive efficient algorithms were still in dire need.In the paper,the compositive orthogonal—matching pursuit algorithm,which was proved the efficiency in practice,and the compositive GA-FCM(Genetic Algorithms and Fuzzy C-Means algorithm) were introduced to segment the typhoon cloud systems.Although the veracity is needed to be improved,the algorithm could efficiency segment the main typhoon cloud systems,and can satisfy the forecast and monitoring of TCs well in the meteorological applications.

Assessing the performance of a prognostic and a diagnostic cloud scheme using single column model simulations of TWP–ICE

AbstractSingle column model simulations using the UK Met Office Unified Model, as used in the Australian Community Climate Earth System Simulator, are presented for the Tropical Warm Pool–International Cloud Experiment (TWP–ICE) field study. Two formulations for the representation of clouds are compared with the extensive observations taken during the campaign, giving insight into the ability of the model to simulate tropical cloud systems. During the active monsoon phase the modelled cloud cover has a stronger dependence on relative humidity than the observations. Observed ice cloud properties during the suppressed monsoon period show that the ice water content is significantly underestimated in the simulations. The profiles of modelled ice fall speeds are faster than those observed in the levels above 12 km, implying that the observations have smaller sized particles in larger concentrations than the models. Both simulations show similar errors in the diurnal cycle of relative humidity during the active monsoon phase, suggesting that the error is less sensitive to the choice of cloud scheme and rather is driven by the convection scheme. However, during the times of suppressed convection the relative humidity error is different between the simulations, with congestus convection drying the environment too much, particularly in the prognostic cloud‐scheme simulation. This result shows that the choice of cloud scheme and the way that the cloud and convection schemes interact plays a role in the temperature and moisture errors during the suppressed monsoon phase, which will impact the three‐dimensional model simulations of tropical variability. Copyright © 2011 Royal Meteorological Society

The High-Altitude MMIC Sounding Radiometer for the Global Hawk Unmanned Aerial Vehicle: Instrument Description and Performance

The Jet Propulsion Laboratory's High-Altitude Monolithic Microwave Integrated Circuit (MMIC) Sounding Radiometer (HAMSR) is a 25-channel cross-track scanning microwave sounder with channels near the 60- and 118-GHz oxygen lines and the 183-GHz water-vapor line. It has previously participated in three hurricane field campaigns, namely, CAMEX-4 (2001), Tropical Cloud Systems and Processes (2005), and NASA African Monsoon Multidisciplinary Analyses (2006). The HAMSR instrument was recently extensively upgraded for the deployment on the Global Hawk (GH) unmanned aerial vehicle platform. One of the major upgrades is the addition of a front-end low-noise amplifier, developed by JPL, to the 183-GHz channel which reduces the noise in this channel to less than 0.1 K at the sensor resolution (~2 km). This will enable HAMSR to observe much smaller scale water-vapor features. Another major upgrade is an enhanced data system that provides onboard science processing capability and real-time data access. HAMSR has been well characterized, including passband characterization, along-scan bias characterization, and calibrated noise-performance characterization. The absolute calibration is determined in-flight and has been estimated to be better than 1.5 K from previous campaigns. In 2010, HAMSR participated in the NASA Genesis and Rapid Intensification Processes campaign on the GH to study tropical cyclone genesis and rapid intensification. HAMSR-derived products include observations of the atmospheric state through retrievals of temperature, water-vapor, and cloud-liquid-water profiles. Other products include convective intensity, precipitation content, and 3-D storm structure.

Impacts of mineral dust on the vertical structure of precipitation

Characteristics of precipitation vertical structure in deep convective cloud systems are compared using measurements from sensors on board multiple satellites over the equatorial Atlantic under dust‐laden and dust‐free conditions. Both case and statistical studies consistently show that the impacts of mineral dust on tropical cloud and precipitation systems are highly dependent on rain type. For convective rain, the pattern of rainfall profiles above the dust layer is mainly determined by updraft intensity. For similar updraft intensity, the convective rain at and below the dust layer exhibits enhancement of the breakup process of raindrops due to mineral dust and associated warm air with strong wind shear. For stratiform rain, dust‐induced microphysical effects are more evident. For similar storm height (or the rain top), precipitation at altitudes above 6 km is enhanced under dust‐laden conditions, suggesting an enhancement of ice heterogeneous nucleation. For warm rain with similar storm height, precipitation is systematically weaker under dust‐laden conditions. Furthermore, the ratio of precipitation water to the total amount of atmospheric hydrometeors is smaller under dust‐laden conditions. Our study suggests that mineral dust exhibits a second aerosol indirect effect on cloud and precipitation systems.

Simulations of microwave brightness temperatures at AMSU-B frequencies over a 3D convective cloud system

Numerical simulations have been carried out to understand the effects of clouds associated with a tropical deep convective cloud system on the Advanced Microwave Sensor Unit-B (AMSU-B) channels at 89, 150, 183.3 ± 7, 183.3 ± 3, and 183.3 ± 1 GHz. The hydrometeor profiles including cloud liquid water, cloud ice, snow, graupel, and rain water for a deep convective cloud system simulated by a realistic dynamical cloud model, the Goddard Cumulus Ensemble model, have been input to a Vector Discrete Ordinate Radiative Transfer model to simulate the nadir down-looking microwave brightness temperatures at the top of the atmosphere. It is found that the AMSU-B channels have large brightness temperature depressions occurring over the clouds with large ice water paths. Moreover, for the three water vapour sounding frequencies around 183.3 GHz, the frequencies broader and further away from the centre of the water vapour absorption line show stronger depressions. The three water vapour channels, particularly the channels closer to the absorption line centre, essentially have negligible influence from liquid water. However, the window frequencies at 89 and 150 GHz have distinct influence from liquid water, particularly the 150 GHz, although they are also strongly influenced by frozen hydrometeors. The AMSU-B frequencies at 150 GHz and water vapour channels of 183.3 ± 7 and 183.3 ± 3 GHz are sensitive to cirrus clouds with total ice water paths above 0.1–0.2 kg m−2. The influence of deep convective clouds and thick cirrus clouds on the AMSU-B water vapour channels demonstrates that they have a potential to estimate ice water paths in thick cirrus clouds and in the upper parts of deep convective clouds, which can complement the retrievals from the 89 and 150 GHz channels.

Simulation and Interpretation of the Genesis of Tropical Storm Gert (2005) as Part of the NASA Tropical Cloud Systems and Processes Experiment

Abstract Several hypotheses have been put forward for the mechanisms of generation of surface circulation associated with tropical cyclones. This paper examines high-resolution simulations of Tropical Storm Gert (2005), which formed in the Gulf of Mexico during NASA’s Tropical Cloud Systems and Processes Experiment, to investigate the development of low-level circulation and its relationship to the precipitation evolution. Two simulations are examined: one that better matches available observations but underpredicts the storm’s minimum sea level pressure and a second one that somewhat overintensifies the storm but provides a set of simulations that encapsulates the overall genesis and development characteristics of the observed storm. The roles of convective and stratiform precipitation processes within the mesoscale precipitation systems that formed Gert are discussed. During 21–25 July, two episodes of convective system development occurred. In each, precipitation system evolution was characterized by intense and deep convective upward motions followed by increasing stratiform-type vertical motions (upper-level ascent, low-level descent). Potential vorticity (PV) in convective regions was strongest at low levels while stratiform-region PV was strongest at midlevels, suggesting that convective processes acted to spin up lower levels prior to the spinup of middle levels by stratiform processes. Intense vortical hot towers (VHTs) were prominent features of the low-level cyclonic vorticity field. The most prominent PV anomalies persisted more than 6 h and were often associated with localized minima in the sea level pressure field. A gradual aggregation of the cyclonic PV occurred as existing VHTs near the center continually merged with new VHTs, gradually increasing the mean vorticity near the center. Nearly concurrently with this VHT-induced development, stratiform precipitation processes strongly enhanced the mean inflow and convergence at middle levels, rapidly increasing the midlevel vorticity. However, the stratiform vertical motion profile is such that while it increases midlevel vorticity, it decreases vorticity near the surface as a result of low-level divergence. Consequently, the results suggest that while stratiform precipitation regions may significantly increase cyclonic circulation at midlevels, convective vortex enhancement at low to midlevels is likely necessary for genesis.

Multiscale Observations of Hurricane Dennis (2005): The Effects of Hot Towers on Rapid Intensification

Abstract A synthesis of remote sensing and in situ observations throughout the life cycle of Hurricane Dennis (2005) during the NASA Tropical Cloud Systems and Processes (TCSP) experiment is presented. Measurements from the ER-2 Doppler radar (EDOP), the Advanced Microwave Sounding Unit (AMSU), airborne radiometer, and flight-level instruments are used to provide a multiscale examination of the storm. The main focus is an episode of deep convective bursts (“hot towers”) occurring during a mature stage of the storm and preceding a period of rapid intensification (11-hPa pressure drop in 1 h 35 min). The vigorous hot towers penetrated to 16-km height, had maximum updrafts of 20 m s−1 at 12–14-km height, and possessed a strong transverse circulation through the core of the convection. Significant downdrafts (maximum of 10–12 m s−1) on the flanks of the updrafts were observed, with their cumulative effects hypothesized to result in the observed increases in the warm core. In one ER-2 overpass, subsidence was transported toward the eye by 15–20 m s−1 inflow occurring over a deep layer (0.5–10 km) coincident with a hot tower. Fourier analysis of the AMSU satellite measurements revealed a large shift in the storm’s warm core structure, from asymmetric to axisymmetric, ∼12 h after the convective bursts began. In addition, flight-level wind calculations of the axisymmetric tangential velocity and inertial stability showed a contraction of the maximum winds and an increase in the stiffness of the vortex, respectively, after the EDOP observations. The multiscale observations presented here reveal unique, ultra-high-resolution details of hot towers and their coupling to the parent vortex, the balanced dynamics of which can be generally explained by the axisymmetrization and efficiency theories.

Cloud-System-Resolving Model Simulations of Tropical Cloud Systems Observed during the Tropical Warm Pool-International Cloud Experiment

Abstract Nested cloud-system-resolving model simulations of tropical convective clouds observed during the recent Tropical Warm Pool-International Cloud Experiment (TWP-ICE) are conducted using the Weather Research and Forecasting (WRF) model. The WRF model is configured with a highest-resolving domain that uses 1.3-km grid spacing and is centered over Darwin, Australia. The performance of the model in simulating two different convective regimes observed during TWP-ICE is considered. The first regime is characteristic of the active monsoon, which features widespread cloud cover that is similar to maritime convection. The second regime is a monsoon break, which contains intense localized systems that are representative of diurnally forced continental convection. Many aspects of the model performance are considered, including their sensitivity to physical parameterizations and initialization time, and the spatial statistics of rainfall accumulations and the rain-rate distribution. While the simulations highlight many challenges and difficulties in correctly modeling the convection in the two regimes, they show that provided the mesoscale environment is adequately reproduced by the model, the statistics of the simulated rainfall agrees reasonably well with the observations.

全球雲解像モデルによる気候変動予測

We have developed a new type of atmospheric general circulation model, “global cloud-resolving model” which covers the globe with a mesh size of a few kilometers. This model enables us to directly simulate the multi-scale structure of the cloud systems of the Earth from meso-scale to planetary scale cloud disturbances. In particular, tropical cloud systems including Madden-Julian Oscillation and tropical cyclogenesis are realistically simulated by the global cloud-resolving model. It is contrasted with current atmospheric general circulation models that are used for climate projections, because they cannot simulate realistic tropical cloud disturbances in general. The global cloud-resolving model not only reproduces the geographical distribution of cloud-precipitation systems and their diurnal to intra-seasonal variabilities, but also more physically reproduces cloud properties by explicitly implementing cloud microphysical processes. It is expected that the global cloud-resolving model will play a major role in climate simulations, and that future climate projections will be more reliable than current approaches.